Nice job on the face studies. I’m far from being able to achieve this at my level, but actually gave that a shot while doodling yesterday and struggled hardcore 😂
I think it cones down to having a very clear and literal idea of the subject’s skeletal structure, placement and shape of features.
In these I think you have to be very careful as tiny changes make a big difference. For example in the top left one the shape of the forehead looks different and the placement of the ear suggests up tilt of the head while the jaw remains at the same level as the head next to it suggesting there is no tilt
Apart from that being careful with the distance between nostrils as in one head the nose seams bigger, especially since it’s stylistically reduced in detail, it makes a big difference in my opinion
That would be my two cents, but overall these look really good and I think the style is pretty cool too
Cheers